In the Bayer process for alumina production, a concentrated sodium aluminate solution is produced by grinding and digesting bauxite in a caustic solution, usually under conditions of elevated temperature and pressure. After clarification of the slurry, the concentrated sodium aluminate solution is cooled and seeded with gibbsite crystals, causing gibbsite to crystallise from solution. The gibbsite is calcined to produce alumina, while the depleted (or “spent”) liquor is recycled to digest more bauxite.
During digestion, some of the caustic is consumed in undesirable reactions with impurities within the bauxite, reducing the liquor's productivity. One of the most significant of these reactions results in the formation of sodium carbonate, arising from the dissolution of inorganic carbonates within the mineral phases present, or from the thermal and oxidative degradation reactions of organic compounds. Unless controlled, with each cycle of the liquor through the process the sodium carbonate concentration would continue to rise, with a corresponding reduction in the liquor's ability to digest gibbsite or boehmite from the bauxite.
The most common technique for controlling the sodium carbonate concentration in Bayer process liquors is to causticise using either quicklime or slaked lime. This process can be carried out either within the digestion circuit itself (by introducing lime with the bauxite), or more commonly, as a side-stream process. The addition of lime directly with bauxite is not common except where lime is required to control other impurities (such as titanium or phosphorus), because the very concentrated liquors contribute to poor efficiency. Unless the temperature is very high, most of the lime undergoes side-reactions with the aluminate in solution to yield calcium aluminate species, particularly tricalcium aluminate (TCA, often also referred to as C3A in the cement industry).
In the more prevalent side-stream causticisation, a dilute liquor stream (usually taken from one of the mud washing stages) is reacted with a slaked lime slurry, generally at close to the atmospheric boiling point of the combined liquor. Alternatively, the slurry is sometimes added directly to the mud washer. The amount of sodium carbonate converted and the efficiency of lime utilisation are dependent upon many variables, but in most refineries, the lime efficiency is in the vicinity of 50 to 70%.
In the alumina industry it is common to refer to a Bayer liquor's carbonate impurity level in terms of the caustic to soda ratio, or ‘C/S’. Here, ‘C’ refers to the sum of the concentrations of sodium aluminate and sodium hydroxide, expressed as the equivalent concentration of sodium carbonate. The ‘S’ concentration refers to the sum of ‘C’ and the actual sodium carbonate concentration, this sum once again being expressed as the equivalent concentration of sodium carbonate. It can be seen from this that a fully causticised (carbonate-free) Bayer process liquor will possess a C/S ratio of 1.00. Typically, the C/S ratio of the concentrated liquor stream in many alumina refineries is in the range 0.8 to 0.85. C/S ratios higher than this are difficult to achieve, because causticisation processes in current use are incapable of fully removing all of the sodium carbonate in the liquor streams fed to them. For example, a liquor with an S concentration of 135 g/L will typically only causticise to a C/S ratio of about 0.890. This limitation arises because the traditional implementation of the causticisation reaction with slaked lime is controlled by a number of complex equilibria, including a competing reaction involving the aluminate ion in which TCA is formed.
By contrast, the causticisation reaction of pure mixed solutions of sodium carbonate and sodium hydroxide with slaked lime is quite simple. The final concentration of hydroxide and carbonate ions is a function of the activities of the various ionic species present, in equilibrium with the solid phases calcium hydroxide and calcium carbonate. The reaction can be described by the following equation:Ca(OH)2+Na2CO3⇄CaCO3+2NaOH  (1)
It has generally been assumed that the above reaction also applies when causticisation is performed in Bayer process liquors. However, it has been known for some time that calcium hydroxide reacts readily with the aluminate ion, ostensibly to form TCA. The formation of TCA is commonly held to occur via one or both of two mechanisms: a simultaneous competitive reaction in which the calcium hydroxide reacts directly with the aluminate ion to form TCA [Chaplin, N. T., Light Metals (1971), 47-61], or a “reversion” reaction in which the calcium carbonate formed during causticisation reacts with the aluminate. However, some authors have suggested that in Bayer liquors causticisation occurs via a “hydrated tricalcium aluminate intermediate” [Young, R. C., Light Metals (1982), 97-117] or a “carboaluminate” phase [Lectard, A; Travaux ICSOBA, 12(17), (1982), 149-156] and that TCA forms as this material ages.
Irrespective of the mechanism proposed, causticisation as practised in the Bayer process has been inefficient in terms of the C/S achieved, and in the efficiency of lime use. Furthermore, poor efficiency of lime utilisation has also meant that quite considerable amounts of aluminate ions are consumed in the formation of TCA. This can represent a substantial loss of alumina production.
A number of causticisation processes have been proposed over the years aimed at improved lime efficiency. However, these processes are generally of limited value in that they are restricted to low ‘S’ concentration wash liquors, requiring large flows to be processed if sufficient mass of sodium carbonate is to be converted to compensate for the carbonate input to the refinery. In U.S. Pat. No. 2,992,893 a process is disclosed in which the thickened mud from a final mud washing stage was causticised, and then reacted with supplementary sodium carbonate to recover some of the alumina lost in the formation of TCA. The causticised liquor was then used in the mud washing stages. Apart from the ‘S’ concentration limitation, this process is not ideal in that a substantial proportion of the causticised liquor is lost with the red mud residue.
An improvement over this process is described in U.S. Pat. No. 3,120,996 in which causticisation is performed in a first stage washer, supplemented by further lime addition to a third stage washer. Higher lime efficiencies were achieved (approximately 85 to 95%), but only in quite dilute washer streams (80 g/L ‘S’), and the achievable C/S ratio or the causticised liquor was not very high.
A later development disclosed in U.S. Pat. No. 3,210,155 involves the direct slaking of quicklime in a clarified wash liquor that had been heated to 100° C. After reaction, the slurry was then mixed with further wash liquor to encourage the reaction of TCA with sodium carbonate, and so recover alumina. While high C/S ratios were claimed with this process, it was restricted to wash streams with ‘S’ concentrations of approximately 15 to 40 g/L.
Another process was developed in Hungary in the 1980s by Baksa et al as disclosed in U.S. Pat. No. 4,486,393. In this process, a red mud slurry from one of the washing stages was heated and fed to a reaction vessel with excess lime slurry. Apart from the “normal” causticisation afforded in this tank, the excess lime reacted with sodalite and cancrinite desilication products to form a calcium hydrogarnet, releasing sodium hydroxide. The discharge from this vessel was then fed to a second vessel, and further reacted with a sodium carbonate solution. This solution was obtained by salting out sodium carbonate from concentrated solutions elsewhere in the plant. The reaction of sodium carbonate with either the hydrogarnet or “hydrated” calcium aluminate resulted in the recovery of alumina and some caustic, although this step tended to reverse the gains made by formation of the hydrogarnet species. While an improvement over the basic causticisation principle, lime and alumina losses through the formation of TCA arc still substantial, and the achieved C/S is still limited by the carbonate/hydroxide equilibrium reaction. Furthermore, efficiency deteriorates badly if low ‘S’ concentration washer streams are not utilised.
In summary, it can be seen that the prior art causticisation methods suffer from deficiencies both in the extent to which Bayer process liquors can be causticised (i.e. the maximum C/S that can be achieved), and the efficiency with which lime is utilised to effect this causticisation. By virtue or their poor lime utilisation efficiency, these processes result in the loss of aluminate from solution, thereby reducing the alumina refinery's productivity. Further, the prior art methods are limited with respect to the concentration of the solutions that can be causticised, becoming very inefficient with liquors approaching typical first stage mud washing liquors, or mud settler overflow liquors.